1. Field of the Invention
This invention relates to the field of a digital image duplicator, such as a digital copying machine, facsimile, etc., and in particular to a method and an apparatus for improving the quality of an output image of the digital image duplicator.
2. Discussion of the Related Art
In a field of a digital image duplication system, such as a digital copying machine or a facsimile machine, one important goal is to improve a quality of a duplicate image of an original document. For this improvement, an appropriate image data processing based on the characteristics of the input image, i.e., whether the input image is a binary text image, a full-color photographic image, or a gray scale image, must be provided. Stated another way, different data processings based on differences in the image region in the original image is required so as to accomplish a high quality image duplication.
In a case that the input image is photographic image data, it is important to prevent the generation of moire noise, which appears as a result of interference between a spatial periodic element of a gradation in the original image and a dither matrix pattern for reproducing the gradations of the photographic image with the digital duplicator. To prevent the appearance of moire noise, a smoothing process utilizing a low pass filter on the original image to control the spatial periodic elements of the original may be provided.
On the other hand, if the input image is binary image data, typically represented by a text region in the original document, the input image will have high frequency elements of spatial periodicity in the image data at edge portions of characters in the text region. These high-frequency elements are decreased with the smoothing process and, as a result, the duplicated image becomes blurred. To prevent this problem which occurs when the input image is text data, it is important to enhance the edge portion to improve a contrast in the duplicated image. To achieve this, a MTF (Modulation Transfer Function) correction process is provided to enhance the edge portions of the image data. However, the MTF correction process sometimes generates a side effect with respect to low density pixels in a photographic region or gray scale region. The side effect is that some roughness in the low contrast gradation area may be generated. As a result, the conventional MTF correction does not provide satisfactory results if the input image is a photograph or has a display of a gray scale.
To provide an adequate image data processing, there is some related arts in this technical field. Japanese Patent Laid-Open Patent Application 3-88478(1991) discloses an apparatus with an adaptive image data processing in a digital image duplicator, which controls a smoothing filter and an edge detection filter which utilize a smoothing conversion table and an edge conversion table. This apparatus controls a processing parameter based on the classification result of an image region of input image data, such as for a photographic document or for a text document. However, the above-mentioned apparatus is not sufficient to utilize a reference pixel density data itself for controlling the MTF correction process for the adaptive image data processing.
Another apparatus is presented by Japanese Patent Laid-Open Patent Application 6-152959(1994) by the applicant of the present invention. This publication discloses an apparatus directed to an adaptive image data processing using an edge amount, with an arithmetic processing of the reference image data for controlling a smoothing process and a MTF correction process. However, this apparatus requires two kinds of circuits for detecting the edge amount of the reference pixel and for processing a MTF correction using the edge amount. These two circuits require a plurality of multipliers. As a result, this apparatus requires a relatively large scale of hardware implementation, which is costly. Furthermore, this apparatus does not utilize a reference pixel density data for controlling the adaptive image data processing.